1. Field of the Invention
The present invention relates to a gate signal generator for a thyristor valve used in a static var compensator (to be referred to as an SVC hereinafter).
2. Description of the Related Art
FIG. 1 shows an arrangement of the SVC for controlling a current flowing through a reactor, using thyristor valves each having a plurality of anti-parallel-connected thyristors as switching elements. Referring to FIG. 1, reference numeral 1 denotes a power tranformer; 2A and 2B, reactors; 3U and 3X, thyristor valves; 4U and 4X, U- and X-phase forward voltage detectors for thyristor valves; CT, a current transformer for detecting a thyristor valve current; 11, a U-and X-phase current detector; and FVU and FVX, forward voltage signals.
FIG. 2 shows a conventional gate signal generator. Referring to FIG. 2, reference numerals 5U and 5X denote normal gate circuits each having an AND gate. The circuits generate gate signals GPU and GPX in accordance with logical AND signals of phase control signals PHSU and PHSX from an SVC controller (not shown) and forward voltage detection signals FVU and FVX from thyristor valves. FIG. 3 is a timing chart showing the signals from the above circuits and voltage waveform V across through the thyristor valves shown in FIG. 1.
In the conventional gate signal generator shown in FIG. 2, when the thyristor valve is fired near a zero-crossing point of the voltage across the thyristor valve terminals (to be referred to as .alpha.=0.degree. firing hereinafter) due to an erroneous pulse, a noise pulse, or the like, the following problems are posed.
Referring to FIG. 3, when firing occurs .alpha.=0.degree. and time t1 due to an erroneous pulse, a current including transient direct current components is caused to flow since the thyristor valves are connected in series with reactors. Resistive components of an electric power system including the reactors and the voltage transformer are much smaller than their reactive components. Therefore, a time interval .DELTA.tR, while a reverse voltage is applied to the thyristor valve after the current conduction, is small. When the reverse voltage time interval .DELTA.tR is short, some thyristor elements are turned off and others are not turned off due to variations in the turn-off characteristics of respective elements. Since a plurality of thyristor elements are connected in series in order to obtain a high voltage rating thyristor valve, a forward voltage immediately after the reverse voltage time interval is applied only to the turned-off thyristor elements. Therefore, when a voltage higher than the withstand voltage is applied to the valve at time tB, the turned-off thyristor elements are broken down, thus possibly damaging the entire thyristor valve.